Method for the production of pure iron, and iron carbon alloys including carbon and alloy steel



Sept. 3, 1957 v. s. ARATA METHOD FOR THE PRODUCTION OF PURE IRON. ANDIRON CARBON ALLOYS INCLUDING CARBON AND ALLOY STEEL 3 Sheets-Sheet 1Filed Feb. 9. 1954 FIG.

ATTORNEYS Sept. 3, 1957 v. s. ARATA 2,805,142

METHOD FOR THE PRODUCTION OF PURE IRON, AND IRON CARBON ALLOYS INCLUDINGCARBON AND ALLOY STEEL Filed Feb. 9, 1954 3 Sheets-Sheet 2 v INVENTOR.Vincenzo 8. Arch:

"MM uwgm mam S um a ATTORNfiXS V. SYARATA Sept. 3, 1957 2,805,142 METHODFOR THE PRODUCTION OF PURE IRON, AND IRON CARBON ALLOYS INCLUDING CARBONAND ALLOY STEEL 3 Sheets-Sheet 3 Filed Feb. 9, 1954 "HI HI Vincenzo S.Arafu ATTORNE S Unite Stats METHOD FOR THE PRODUCTIQN F PURE IRGN,

AND IRON CGN ALLDYS iNCLUDlNG CAR- EON AND ALLSY STEEL Vincenzo StefanoArata, New York, N. 1., assignor to James E. Brassert, New York, N. Y.

My present invention relates to the commercial production of pure ironand iron-carbon alloys, including carbon and alloy steel, directly fromore and carbonaceous material. In using the terms directly or directprocess in the succeeding text, I mean that the process does not involvethe production and handling of intermediate materials such as coke andpig iron, and that the iron or steel is produced directly in the moltenstate.

In the past, single charges of ore and carbonaceous reducing agent havebeen converted into steel in the electric furnace, but the methods thathave been used up to now have had one of two principal disadvantageswhich have prevented their coming into general use. The first of thesehas been'the high estimated capital and operating costs of full-scaleplants of this type. These high estimated costs have in turn been due tothe operation of the plant purely on a batch process. The salient factorcontributing to high estimated operating costs has been the high powerconsumption of 2500 kwh./metric ton or more and similarly the largestfactor in capital cost has been the high cost of power-generatingfacilities for a given annual steel ingot capacity. These factors havetended to discourage the installation of plants employing the directprocess in many localities.

My present invention embodies a method of treatment whereby powerconsumption can be decreased one-third to one-half resulting in adecrease almost as great in total cost, and a proportionate decrease incapital cost of generating facilities. Furthermore, according to mypresent invention, steels can be produced to the required analyses on aregular basis with a minimum number of olf heats.

According to one method for the production ofpure iron, of my inventionas disclosed in United States Patent No. 2,747,985, the ore is finelycrushed and intimately mixed with the required flux and with adetermined quantity of finely divided carbonaceous reducing material.This quantity is determined so that after allowing for carbon lost inthe flue dust escaping with the top gases, sufficient carbon is left inthe charge to reduce between 60 and 90 percent of the iron content ofthe charge as calculated stoichiometrically. I normally use thefollowing equations for this type of stoichiometric calculation:

In the production of steel in contrast to the production of pure iron, Idetermine the quantity of carbon charged analogously, except that Iprovide sufficient carbon to reduce approximately 85-100 percent of theiron content of the charge. According to my previous method ofoperation, all of the materials were charged, melted, and refinedbatchwise in the electric furnace, and the manner of accomplishing thistogether with a physical and chemi-. cal description of the process isdescribed for the case of pure iron in the above-mentioned patentapplication. The production of steel is analogous except for the differences in carbon content of the charge noted above.

Numerous attempts have been made in the past to make the direct processfully continuous, in order to reduce capital and operating costs. I havefound in many years of practice that fully continuous production isinconsistent with regular analysis and reliable quality of steelproduced. Therefore the present process can be considered to be a batchprocess with respect to the refining period and to the cycle of a singleelectric furnace, but it can be considered to be continuous with respectto the charging of raw materials and the production of steel in theplant as a whole, as well as with respect to the utilization of gas fromeach individual furnace. The elements of the plant are connected in sucha way that the operation of the preheating chambers is continuous, andthe effect of the preheating (and partial reduction) step is to decreasepower consumption from 2500 kwh./ metric ton to 2000 kwh./metric ton orless. My process therefore involves a number of electric furnaces and anumber of preheating chambers. With respect to the flow of solidmaterials, each electric furnace is connected in parallel with at leastone other electric furnace and in series with at least one preheatingchamber. With respect to the flow of gas, the same is true, except thatin the majority of cases all of the electric furnaces will be inparallel with each other and in series with all of the preheatingchambers, which in turn will be parallel with each other.

Before describing the operation of my process by a concrete example, itmay be advisable to point out several ways in which the carrying out ofmy process will deviate from and represent an improvement upon 'pastmethods of conducting the direct process.

First, in the making of steel from ore and carbonaceous material by thedirect process, the most critical period is the refining period duringwhich the heat is adjusted with respect to its final carbon content, andthe sulphur and phosphorous contents are brought down to the correctlevels. Most coals contain appreciable sulphur and this element is alwaythe most diflicult to eliminate in the direct process. This is due tothe fact that the direct process employs a slag high in FeO and theelimination of sulphur requires a strongly basic slag low in FeO.Therefore, frequently it is necessary to remove the first slag andreplace it by a strongly basic slag rather low in FeO. This procedurecannot be efficiently or reliably carried out while excess carbonaceousmaterial (usually coal or coke) remains on top of the slag, as has beenattempted in the past, since the removal of the first slag would bringthe carbon of the coal in contact with the metal and carburizc it sothat the steel would not show the correct tapping analysis in carbon.Therefore it is not feasible to charge excess carbon to the furnace andstill maintain a regular and controlled analysis of the steel produced.Accordingly, in my process, after allowing for carbon lost in the fluedust and for carbon going into the steel, 1 only charge sufficientcarbon to reduce -100 percent of the iron content of the raw materials,and I conduct the refining process batchwise so as to provide theopportunity to charge a second slag for removal of impurities, includingsulphur and phosphorous.

Second, if it is attempted to produce steel continuously in the electricfurnace by continuously charging fresh materials, it will be impossibleto refine the molten metal into steel in the same furnace; this is dueto the difficulty of separating the metal being refined from theincoming material. The incoming, impure materials tend to increase theoxygen and sulphur contents of the metal bath, to introduce dirt andother contaminants, and to vary the analysis of the product. Suchcontinuous operation has been tried in the past, and has re sulted inirregular analysis of the steel produced. There fore, in the presentinvention the charging cycle of 3 each furnace is discontinuous andrepresents only a portion of the total heat time, and the refining iscarried out batchwise.

Third, those furnaces, which'have operated continuously to produce steelby the direct processhave been required to hold high temperature slagsand iron continuously and this has resulted in high refractory lossesand frequent breakouts, which have seriously inter rupted operations andin many actual cases rendered the process impractical. According to mycycle of operation, a portion of every cycle of each furnace after eachheat is allocated for repairingthe walls and the bottom of the furnace,as is normally done in a batch process; this gives the operator time toanticipate and prevent costly breakouts and to maintain a satisfactoryfurnace lining. Furthermore, in ,my cycleof operations, which lastsnormally five hours, the reduction operation requires approximately fourhours, and during this period only the locations adjacent to the arcsare at high'temperature. Accordingly, reduction proceeds throughout thegreater part of the charge'at temperatures less than 1200510., and thewalls and roof remain .Only during the refining period, which requiresabout half an hour of the total cycle, is the temperature 'of thematerials adjacent to the walls raised above the-melting temperature ofsteel. Thisfactor has such an important bearing on the life of thelining as to spell Ethedifierence betweenfeasibility andnon-practicability of the direct steel process, and it is a basicfeature of my-present invention.

. I will now give a description 'of a typical plant in which theelectric furnaces are operated in parallel and according to a definitecycle, in order to give a clear picture of its operation.

Such a plant will consist of a number of units, as hereinafter morespecifically defined, each unit including as elements electric furnacesconnected in series with one or more preheating chambers. The purpose ofthe preheating chambers is to preheat and in most cases also topartially reduce the ore. In all cases, the electric furnaces of eaclrunit Willbe connected in parallel with each other with respect to theflow of gas and of solid materials, and the preheating chambers of eachunit will be similarly connected in parallel with each other. Thepreheating (partial reduction) chambers in each unit may be of var-ionstypes (for example, rotary kiln or shaft furnace) and may functiondifferently with respectto their physical and chemical treatment of theraw materials. For example, each unit may'include'two parallel sets ofkilns ofwhich .each'set is made'up of two kilns in series so thatpreheating and roasting of the ore and a degree of coking of thecarbonaceous material (by which a large portion of the volatilematerials is rapidly eliminated) take placein the'first kilos of theseries, while partial reduction and additional Ipreheating take place inthe second kilns of the series. However, partial reduction andpreheating in the second kilns should never go so far as to result insintering of the iron-containing materials, i. e., in practice thetemperature of the material leaving the second kilns should be600-1000"C. In other plants operating with different types of carbonaceousreducing agent, the individual units may consist simply of one or twoparallel kilns into which are charged properly prepared iron'ore andcarbonaceous material and which discharge in parallel into a batteryconsisting of at least two electric furnaces.

The preheating chambers should preferablybc so designed as to permitregulation of the-residence time of the charge materials passing throughthem so that the discharge cycle. of materials from the preheatingchambers into the electric furnace is not rigidly determined by thecharge cycle of the same materials into the chambers. This featureenables the charging and discharging of the several preheating chamberstobe relatively inde pendent of one another and more flexibly adaptableto the electric furnace cycle.

For example, in case the preheating chambers consist of rotary kilns,such regulation can be accomplished by providing control of either thedegree of tilt of the kilns or of their rate of rotation, or both.

The number of units in a large plant is of course adapted to theproduction requirements of the entire plant without'departing from theprinciples followed in a small plant or in an individual unit. It shouldalso be stated that although, with respect to the flow of materials,only the furnaces and preheat chambers in .a single unit may be inparallel with each other, with respect to the flow of gas, all of thefurnaces in the entire plant are preferably connected in parallel, andthe same holds true for all of the preheat units with respect to theflow of gas.

Each individual unit may consist of at'least two and possibly moreelectric arc furnaces'with carbon or graphite electrodes preferably ofthe Soderberg type. The furnaces may be either of the conventionalcylindrical type or rectangular or of any convenient shape; they may beeither tilting or stationary. They need not be-arc furnaces since I mayalso perform my present invention with other types of electric furnace.

I shall now explain my invention in detail with respect to one specificembodiment thereof selected by way of example and with reference to theaccompanying drawings in which:

Fig. '1 is a schematic plan view, partly in section, of a unit forcarrying out my process;

Fig. 2 is a schematic end view, partly in section, of the unit .shown inFig. 1;

Fig. 3 is a schematic side view, partly in sectiomof the unit ,shown'inFig. 1;and

Fig. 4'is a graphical representation of timing relationships betweendifferent phases of the operation of the unit-shownin Fig. 1.

Figs. .13, inc., show a single pair of rotary kilns, 2 and 2 operatingin parallel with each other and in series with a single pair of electricfurnaces, 10 and 10'. This combination represents one of the simplestmethods of carrying out my process. In Fig. 1, horizontal screwconve'yors 1 and 1 charge the material continuously into chutes and fromthese chutes through passageways into kilns 2 and 2'. Such passagewaysshould always be kept full of charge material so as to maintain a sealpreventing the escape of the gases which pass through the kilns. Afterpartial reduction and preheating in the kilns 2 and 2 arecomplete, thematerials pass into a common hopper 4 which, as shown in Fig. 2 has twoexits, 5 and 6, each with it own valve. The chute passing out'of exit5in turn diverges, as shown in Fig. 3 into two chutes, 7 and 8, alsoprovided with individual valves, and through these chutes the materialsfall into furnace 10. The passage of materials through exit 6 intofurnace 10 is entirely analogous, through chutes .7 and 8'.

With respect to the flow ofgas, uptakes 11, provided with appropriatevalves convey the gas evolving from reduction taking place withinfurnace 10 into collector main 9. Uptakes 11' similarly connect furnace10 to collector main 9 which is common to both of the furnaces. (In casethere are more than two furnaces in the unit or in' the plant as awhole, collector main 9 should be common to "all of the furnaces. Ina'ny'c'ase the furnaces 'are operated in cycle so as to maintainasnearly "as possible a steady flow or gas for use elsewhere "in theplant.) According to my process it'must be possible to'shut on thechambers 2 and 2' by means of valves in the passagewaysl3 and 3' betweencollector main 9 and thelcharnbers. Thiss e stt 1 1 Rmat s-t a n th snfiq ias of my process, which permits operating a number of preheating(and partial reduction) chambers in parallel with each other and inseries with a number of electric furnaces also operating in parallelwith each other. Any chamber or furnace unit should be separable at willfrom all of the other units comprising the whole, with respect to theflow of both solids and gases. All of the units of the plant shouldoperate under a slight superpressure which is maintained by valves inpassages 11, 11', 3, 3', or by equivalent valves located, for example,in collector main 9.

In certain cases it might be desirable to maintain combustion in thekilns by adding air. Such air injection is carried out by means ofinjectors 15 and 15 which conduct air to act as a combustion medium forthe gases in passages 3, 3; the flame can of course be regulated to bemore or less oxidizing. The gases are exhausted from the rotary kilns 2,2' through a common passageway 16, and part of the gases may be recycledinto collector main '9. Because of the high temperature of the gasesleaving the electric furnaces 10 and 10, passageways 11 and 11, '9, 3and 3' must be provided with suitable refractory linings.

Fig. 4 is a diagram illustrating a particular method of operating aplant according to my process. The diagram refers to a plant unit havingtwo electric arc furnaces in parallel with each other which are fed bytwo kilns, also in parallel with each other, and it illustrates thenormal cycle of operation of each of the two kilns and each of the twofurnaces. Each furnace operates according to It is understood that thefollowing description is applicable also to a plant having more chambersand furnaces than illustrated and in which the total period and therelation of the various cycles may vary in accordance with thecharacteristics of the charge and the number of chambers and furnacescomprising the plant. At the beginning of a campaign furnaces arebrought up to temperature by melting in them several successive scrapcharges. Afterwards they are ready to receive the first ore-containingcharge. The charge proceeds as follows: several shovelfuls of coal arefirst introduced, for example, into furnace 10, through open door 14(the valves in passages 11, 7 and 8 being closed). The introduction ofcoal serves the purpose of absorbing and eliminating the oxygen of theair which is present in the interior of the furnace prior to openingchutes 7 and 8, and thereby avoiding the slight explosion which mightotherwise occur when the considerable amounts of carbon contained in theentering charge are suddenly brought into contact with the air withinthe furnace. The appearance of flames at the door 14 of the furnaceindicates that gas is present within and, upon this observation, thecharge controlling valves in passages 7 and 8 are opened (valve 5 beingopen and valve 6 closed). Passages 7 and 8 are kept open until chargehas been deposited beneath the electrodes to a thickness of at least onefoot, after which they are again closed. Since the charge coming fromabove is deposited somewhat unevenly, the surface of the charge is thenraked to form a smooth bed beneath the electrodes. On top of this bed alayer of iron scrap several inches thick is charged through the door.Subsequently the electrodes are lowered and as soon as all three of theelectrodes touch the surface of the scrap, power at maximum voltage andminimum amperage is supplied to start the arc. It is necessary to employmaximum voltage to increase the power absorption during the initialperiod when the conductivity of the charge and therefore flow of currentare low. The reason it'is necessary 6 to maintain ahighinitial powerconsumption is that normally the electrodes are regulated so that theyare raised and lowered by increases and decreases of both amperage andtotal power and it is necessary to avoid that the electrodes should sinkinto the charge due to the small amount of current passing them and thelow power consumed. It should be emphasized that it is impossible tostart the arc unless use is made of the high conductivity of the scraplayer; this layer permits the current to flow and a small bath of moltenmaterial is rapidly formed. However, it should be noted that in somecases the partial reduction operation may have produced suflicientmetallic iron or may have sufliciently raised the temperature of thecharge so that, due to the high conductivity of the charge entering theelectric furnace the are will start spontaneously; in this case thecharging of the scrap layer can bedispensed with. Normally nodifficulties present themselves in this phase, but in case one of theelectrodes should tend to sink into the charge, a small amount of scrapor reduced iron should be added through door 14 in such a way as to makea good contact. After the proper steps have been taken to insure thatthe current and power are balanced between the three electrodes the orebegins to be reduced in quantity and the gases appear at door 14.;At'this point'door 14 is closed and passages 7 and 8 and 11 are opened.From time to time the amperage is increased in the normal manner and thevoltage decreased to the minimum point consistent with good operationsince it is advisable to transfer to the charge as much heat as possible(this transfer being representedaccording to Joules equation W=I R).

From time to time either of passages 7 or 8 can be closed for a while toequalize the level of the charge in both ends of the furnace and topermit observing the operation of the furnace through peepholes.

After completion of two hours of the charge cycle'of furnace 10,charging of furnace 10 is commenced by opening valve 6 and in thismanner kilns 2 and 2' will be supplying the entire charge requirementsof both furnaces 10 and 10' for a period of one hour.' Three hours afterthe beginning of the charge cycle of furnace 10, valve 5 is closed sothatthe charge proceeds through valve 6 only into furnace 10 so as tocomplete the charge cycle of furnace 10. After stopping the charging offurnace 10, reduction continues in this furnace for approximately onehour. Then gas uptake passagell is closed and door 14 may be openedslightly permitting the gases to blow out. The bath is then agitatedwith a stirrer. When no further gas appears the reduction cycle iscomplete. Door 14 can now be fully opened, if necessary, in order todetermine whether pieces of the charge are adhering to the walls, inwhich case they can be put back into the bath. At this time a testsample is taken to determine the carbon content of the melt. The takingof this test sample is very important at the beginning of the campaignwhen it is desired in succeeding heats to obtain a steel of a definitecarbon content without adjusting the bath by adding a second slag. Bycharging the furnace in accordance with my patent application U. S.Serial No. 286,213, or in accordance with those modifications of thispatent application which are described above as applying to steelmanufacture, it is possible to produce a bath containing a regularlyconstant content of carbon. When operating with high-grade ores of low Pand S content, and with ores containing MnO and of low P and S content,it is recommended that several shovelfuls of or ferro-silicon be addedon top of the slag in order to reduce part of the FeO or MnO content ofthe slag into the metal without increasing the carbon content. This stepis naturally taken only in case the carbon content of the first test isat the correct level or lower; if the melt carbon is higher than therequired final carbon content it is not possible to add ferro-silicon,

' 7 since it is first necessary to reoxidize with a second slag toremove the carbon and-the effect ofa prior addition of ''a reducingmaterial such-as ferro-silicon would be lost.

After the proper additions have "beenmade, slag notch 12 -i s--opened{or if the furnace is a tilting "type, it is tilted) 'and'the slag -istapped. The height of the slag notch-is maintained "at'sue'h'a level 'ina stationary furnace that the final levelof *t-hesteel reaches thebottom ofthe slag notch andthat allot the slag is properly tapped. Thisis -an-easy matter to provide in a plant of this type. After the firstslag has been tapped, a second slag can be charged in the normal manner.Additions of ferromanganese, term-silicon, or other materials are thenmade,--and-the temperature of thesteel is brought to the required-level.Subsequently tapping hole 13 is opened and the metal tapped into ladles.-Fina-lly the walls and if necessary the bottom are repaired withmagnesite or doloniitei-n the regular "manner. *Several shovelfuls 'oflimestone are fed over the bottom and the'furnace is nowready'to-recommenc'e its cycle. The combined refining, tappingra-ndrepairing cycle will take approximately one hour. However, theflexibility of the plant permitsuthe oharging-of furnaee 1 --to hedclayed'or accelerated, or it also-permits thecycledoffurnacelo to beprolonged.

'I he oyc'leaof furnace is entirely analogous to that of furnace 10except for a phase lag of approximately two hours.

.Sinceit is advisable that, withrespect tothe how of gas, all of thepreheating chambers of each unit and normallysofthe entire plant beconnected in --parallel with each other, a certain superpre'ssure-isnecessary'to maintainthe flowof gas-through the-entire system. Thispressure gis estimated at several millimeters of water-column foralunit, and greater for several parallel units. Such superpressure musthe also maintained in the furnaces duringgtherreduction tpartof thecycle, although not in the refiningiperiod when the furnaces-isdisconnected from the remainder .of ithesystem. Therefore it ischaractenistic ;of our process that the individual furnaces duringpthereducing cycle, and the connec-ted gas system as a whole, .he maintainedunder a slight superpressure. If adequate :s'uperpressure cannot :bemaintained under normal conditions, it is necessary to create suchsuperpressure by, for instance,.compressing a portion of the waste gases:and recycling it into the furnaces during the reduction cycle.

In conclusion, thesadvantages to be obtained by practicingihe presently-claimed improvements in all types of ironand steel production :arise.from the combination of cont nuous charging and preheating .and batchrefining. Whereas previously it has not been feasible, practically and.econQmically, to produce steel .on a large scale by the direct process,my claimed improvements will overcome this disability. The :two sets ofdesirable conditions obtainedsby the combinations .of my claims arelisted as follows:

A. Advantages .of continuous operation obtained by thisprocess:

.(i'l) Continuous charging into .and .out of the preheating chambers ofeach unit; :and into the plant .as a whole. o

12) Regular production of metal from the plant as a whole due %to.staggering the tapping :of the individual furnaces-in accordance withthe typicalcycle.

(at) Power consumption .per .ton .of metal .decreased by preheating:and/ or partial reductiomcharges resulting in thezcase of steelproduction in a considerable decrease in :power con ump on p ton as ompae with .2500 kwhJn etric-ton in a batch :Process.

(4) 'E-ulland efiicient use -,of -the;g as'from the electric furnace.plant ,as a whole to preheat :and partially reduce the-char 7 t5)Constant volumecf ga delivered brothe u in the plant.

' 'B. Advantages-of batch refining cycles obtained -hythis process: 7

1*) Regular productionfromore ineachplantunitlof all gradeslof high,medium, low carbon and alloy steel products of-unifor-m analysis :andto-specification.

(2) A long refractory life dne to t-he fact that temperature of furnacewalls is maintained low for aflmajor portion-of each cycle. Thetemperatures only exceed 7 the melting temperatures of the materialsduring the short refining period; Roof temperatures are likewisekeptlow.

(3) A period is allowed during .each cycle forrepairs to the bottom,walls and roof, thereby permitting an enormous increase'in lining life.as compared with .a con.- tinuousrefining operation.

--C. Advantage uniqueto :this process:

A further advantagethat is notcharacterist-ic of either a purelycontinuous or a purely rbatchloperation is that within each unit, if.sufiicient elements are provided, in

the event of aibreakdown of a single preheat chamber or of a singlelfurnace, another sequence .of chamber or furnace operation can besubstituted. Frequently such substitution would not disturb in anydegree the ,continuous features of'the operation. As an example, in anindividual .unitaconsisting of two preheat chambers and three electriciurnaces, the preheat chambers may he so designed that .one can handlethe full rated charge of the unit, and that the .cycles :of :twofurnaces can be staggeredsoas to handle this charge in the event thethird furnace is temporarily shutdown. *Such substitutions provide adegree of flexibility that cannot be obtained in either a batch or acontinuous plant, and the greater average availability reduces .dowhtimepostand over.- head charges, .thus improving theefiicienqy ofoperation.

In the following claims, the term innit refers :to a number .of.electticfurnaces and preheating chambers, ineluding that number .ofelectric furnaces (never less than two a du ual y no m r whi h i o n t ir spectto the flow..of solid raw materials, in series with ,a singlepreheat chamber or i i-series with several preheat chambers connected inparallel with each other, and including the n mber of preheat chambersthus connected in parallel.v T he term battery refers to all of theelectric furnaces in the unit or to ,all of the preheating chambers inthe unit.

I claim:

1. Themethod ,ofoperating aplant for the production of molten pure ironand iron carbon alloys from ores, said plant being of the typeincludingone or more plant units, each unit comprising two or more electricfurnaces connected with each other in parallel with respect to thein-flow ,of ,solidcharge materials and the out-flow of evolved gas andone or more preheating chambers connected in ,sprieswith said furnaceswith respect to said flows, in which the cycle of each individualfurnace includes a reduction period during which the flow of evolvedgases from said furnace is connected so .as to preheat solid chargematerials continuously entering said furnace from one or more of thepreheating chambers by countercurrent flow in such chamber, a batchrefining period during which said furnace is disconnected from the restof the unit'with respect to the flows of solid charge materials andevolved gas, and a repair period during which said furnace isconditioned for the'sncceeding cycle.

2. The method of claim 1 inwhich the solid charge material includes,iron ore and carbonaceous material in aproportion to said ore such thatthe available carbon, after allowances for fiue dnstand alloying, doesnot exceed the amount theoretically suflicient to reduce of the ironcontent of said ore.

3. The method of claim 1 in which the temperature of the furnace wallsand roof is maintained below about 1300 C. duringat least two-thirds ofthe cycle.

4. The method of claim 1 in which a gas pressure of'at'least severalmillimeters of water column gauge, is

maintained in each furnace and in the gas system of the unit connectedthereto during its reduction period.

5. The method of claim 1 in which the preheating of the solid chargematerials in the chambers is conducted at such a temperature or for sucha period of time as to produce, as the solid charge material fed into afurnace at the beginning of a cycle, a material of a conductivitysuflicient to start an electric furnace are.

6. The method of claim 1 in which the preheating of solid chargematerials in the chambers is conducted at 600 C.-1000 C.

7. The method of operating a plant for the production of molten pureiron and iron carbon alloys from ores, said plant being of the typeincluding one or more plant units, each unit comprising two or moreelectric furnaces connected with each other in parallel with respect tothe in-flow of solid charge materials and with respect to the out-flowof evolved gas and one or more preheating chambers connected in serieswith said furnaces with respect to the flow of said charge materials andevolved gas, comprising the steps of partially reducing said chargematerials in said chambers by preheating with said evolved gas to atemperature of 600 C.1000 C., further reducing said charge materials ina single furnace while at the same time charging said single furnacewith said charge materials, disconnecting said single furnace from theunit with respect to the flow of said charge materials and evolved gas,completing the reduction and melting of the charge materials in saidsingle furnace.

References Cited in the file of this patent FOREIGN PATENTS

1. THE METHOD OF OPERATING A PLANT FOR THE PRODUCTION OF MOLTEN PUREIRON AND IRON CARBON ALLOYS FROM ORES, SAID PLANT BEING OF THE TYPEINCLUDING ONE OR MORE PLANT UNITS, EACH UNIT COMPRISING TWO OR MOREELECTRIC FURNACES CONNECTED WITH EACH OTHER IN PARRALLEL WITH RESPECT TOTHE IN-FLOW OF SOLID CHARGE MATERIAL AND THE OUT-FLOW OF EVOLVED GAS ANDONE OR MORE PREHEATING CHAMBERS CONNECTED IN SERIES WITH SAID FURNACESWITH RESPECT TO SAID FLOWS, IN WHICH THE CYCLE OF EACH INDIVIDUALFURNACE INCLUDES A REDUCTION PERIOD DURING WHICH THE FLOW OF EVOLCEDGASES FROM SAID FURNACE IS CONNECTED SO AS TO PREHEAT SOLID CHARGEMATERIALS CONTINOUSLY ENTERING SAID FURNACE FROM ONE OR MORE OF THEPREHEATING CHAMBERS BY CONTERCURRENT FLOW IN SUCH CHAMBER, A BATCHREFINING PERIOD DURING WHICH SAID FURNACE IS DISCONNECTED FROM THE RESTOF THE UNIT WITH RESPECT TO THE FLOWS OF SOLID CHARGE MATERIALS ANDEVOLVED GAS, AND A REPAIR PERIOD DURING WHICH SAID FURNACE ISCONDITIONED FOR THE SUCCEEDING CYCLE.